Was this option added from a new firmware update?
Maybe I cannot remember, but I think I had seen this Fast Encryption option on Android 4.3 before upgrading to KitKat.
Because most manuals and online tutorials on how to encrypt an Android device, does not even mention the Fast Encryption option.
When did this option come about?
I wonder that this came with one of the firmware update bundles...I don't know.
And another question - Most manuals to decrypt the phone are mentioned as the following:
One way: The encryption process is one way. Once encrypted, you cannot undo this. The only way to disable encryption is via factory reset.
But my device, just has DECRYPT DEVICE in order to remove the encryption without doing a Factory Reset. And I did not select the Fast Encryption option either...